The present invention relates to an endoscope objective having a viewing direction that includes an angle of  less than   greater than 0xc2x0 relative to the longitudinal axis of the endoscope, i. e. to an endoscope objective of a so-called inclined-eyepiece endoscope.
Endoscope objectives of this type are commonly known. German Patent DE 35 37 155 A1 discloses an objective comprising of a lens system which forms the image of the object field of the objective in an image plane that is disposed orthogonally on the longitudinal axis of the endoscope.
That known objective comprises a lens system which forms the image of the object field of the objective in an image plane that is disposed orthogonally on the longitudinal axis of the endoscope. The image formed in the image plane is then relayed or imaged, respectively, by an image relay system into a proximal image plane. Between the lenses of the lens system a dual section prism unit is disposed in which the beam is so reflected on two boundary surfaces that the beam is deflected in the prism unit from the desired viewing direction into the longitudinal axis of the endoscope, i. e. into the optical axis of the image relay system.
In accordance with prior art as known from that reference a layer consisting of MgF2 is applied on an outside of the dual section prism unit, through which the beam should not pass, with the beam undergoing total reflection against this layer.
Due to the use of a dual section prism unit the system with an inclined eyepiece for endoscopes, as it is known from the German Patent DE 35 37 155 A1, is complex and hence expensive to manufacture.
With a system known from the U.S. Pat. No. 4,684,224 an optical flat on which a plano-concave lens is cemented is applied on the entrance surface of a prism changing the viewing angle.
On account of the application of this optical flat the known endoscope objective is equally comparatively complex and hence expensive. Moreover, the space required for the accommodation of the optical flat must not be neglected.
It is furthermore known that an air gap is provided instead of the optical flat. However, this is possible only with endoscopes having a comparatively wide diameter whilst the assembly is practically no longer possible with endoscopes having a diameter of 4 mm or less.
The present invention is now based on the problem of improving an endoscope of the claimed general type in such a way that air gaps or prisms having a dual segment structure can be dispensed with on optical flats applied on the entrance surface of the prism.
In accordance with the invention the coating is applied on that surface of the prism unit which faces the glass cover of the endoscope. In distinction from the inclined-eyepiece system known from the German Patent DE 35 37 155 A1 the thin layer, which has a refractive index smaller than the refractive index of the material which the prism unit is made of so that total reflection occurs on the thin layer rather than on air or another glass material, serves a double function:
Firstly, the beam entering the objective passes through the coating, and secondly it serves for total reflection of this beam after it had been reflected on another surface. The coating must hence be made of optically transparent materials. It is particularly preferable that the coating serves at the same time as stratified system reducing reflection.
Compared against the application of an optical flat, the application of an optical coating and particularly a thin layer by means of a common coating process, e. g. by way of vacuum evaporation, is a comparatively simple and low-cost technique. The application of an optical coating having a thickness of a few xcexcm with a single layer is sufficient in order to allow for total reflection with negligibly small losses in reflection.
On account of the use of a thin layer it is no longer necessary to provide an air gap so that a mechanical spacer is not required because one element of the objective can be arranged on the layer of the prism unit. Compared against conventional endoscope objectives of different types, the assembly is hence definitely simplified. And there is no occurrence of losses in absorption, as is the case with reflection on metal layers.
In particular, the element and the prism unit can be cemented to each other.
The element may have a wedge shape, which provides for an additional deflection of the viewing direction and hence permits adjustment of the viewing direction, or the distal window of the endoscope. This distal window may be configured as plano-concave lens in the usual manner.
In another embodiment, the prism unit presents three optically operative surfaces whereof one surface, through which the beam arriving from the object field, enters into the prism unit, includes an angle relative to the longitudinal axis of the endoscope, whilst another surface is disposed in parallel with or at an inclination relative to the longitudinal axis and the third surface is arranged orthogonally on the longitudinal axis.
In this configuration, in particular, a coating having the inventive formation may be applied merely on the first surface whereas the second surface is mirrored.
The aforedescribed configuration is expedient particularly in the case of 60xc2x0 prisms. There the second surface is arranged to be parallel with the longitudinal axis of the endoscope. With the arrangement of the second surface at an angle relative to the longitudinal axis of the endoscope varied angles of the viewing direction are obtained. Other angles can also be achieved with the provision that the wedge shaped element is provided for additional deflection of the viewing direction or that an oblique passage of the optical axis through the entrance window is permitted, i.e. at an angle at variance from 90xc2x0, as will be explained still in the following.
As has been set out above, the wedge shaped element may be disposed on the coating directly.
The coating may consist of several layers having different refractive indices. In such a configuration the lowermost layer may be a low-diffraction layer where total reflection occurs. It is particularly preferred, however, that the lowermost layer is a high-diffraction layer and particularly a layer promoting adherence so that total reflection takes place in the coating. It is also possible in particular that several low-diffraction layers are provided. Total reflection can occur on or within the coating.
It is, of course, equally possible that the coating is a single-layer system including only a single low-diffraction layer.
The viewing angle can moreover also be adjusted by the provision that the passage of light takes place through the lens on the object side at an inclination. The imaging errors induced by the oblique passage of the light can then be compensated in the objective.